Not quite the list I was expecting. 2 and 4 are very close to the same, and #1 is simply a result, not a "reason."
Here's my reasons why tablets won't seriously compete with the iPad.
1: it's a CONVENIENCE device. It's intent is to supplement both your phone and PC, not be "another" PC. If I have to baby it, patch it, tweak it, manage it, etc, it's not convenient.
2: Instant-on/always-on. For a "living room" device like the iPad, that's half it;s use case in my mind. The laptop is a pain, waiting for it to boot, waiting for it to connect, launching apps, by the time I'm ready to use it, I'll have forgotten why i booted it up. Also, it's simply not worth a 3-5 minute boot cycyle (up and down) to do a 20 second search on IMDB, or send a quick e-mail or make a note. The Fact that Full OS hibernate as well is one more drawback, but with an iPad, i know instantly if someone loggs on to chat, sends me a message, or i get some other alert. It's like having my phone in the living room with me to get notices, but I have a larger cleaer interface and don't suffer on a cramped 4" screen.
3: H.264. nuff said. If you're tablet is close in price to the iPad, it won;t have 720p or 1080p video output of H.264 video. Most of them proposed won't even handle H.264 at all. With WebKit and HTML5 fast becoming the new media standard, investment in anything that can't do that today better be a throw-aweay device price... Tablets are not at that price.
4: size and weight. If it has a swivle screen, it's notebook thick. I can slide an iPad into my laptop case, but I can't put 2 laptops in it, nor do I want to. I've not seen a tablet with multi-touch proposed yet that is as think or as light as the iPad, not under $900 anyway...
5: Power cables and batteries. iPad uses the dock connector, and can charge off USB or an outlet anywhere, further these cables are ubiquitous, and if I've forgotten one, either someone has one to loan for 30 minutes to charge, or I can buy one for $5-10 anywhere. Full OS tablets require a brick, and have far less battery life. Even if they can last 10 hours, it's another brick to carry around, and they're not stocked commonly anywhere and use proprietary connectors. A replacement in a pinch is not easy to find, and typically requires a $120 universal charger. This is a real issue if I want to have a charger at the office, and leave another in the car, as well as the one at home. I don;t bring my iPhone cables with me, since I already HAVE one everywhere (3 at home, 1 in each car, 1 on office desk, and a spare in the suitcase just in case) and I only bought one of those seperately and it came with a 110v and a CAR adapter for $10.
6: Wireless N and bluetooth. Almost none of the tablets have WiFi n, which is a huge issue for high def video. The ones that do often lack bluetooth, making is less useful for chat/video chat (and yes, the iPad CAN do video chat, using a simple bluetooth camera).
7: application costs. Even if they make a full OS tablet that solves ALL of the above issues, and at a comperable (within $100) price as an iPad, the fact I'll still need to buy unique applications to run on it, completely seperate from my desktop, means that pad comes burdened with a few hundred dollars in excess costs. Worse, every app I add slows it down and eats the battery, since there's no background app controls, and no reseounce management in the OS designed with the idiea of quick, light, and long life from the ground up. At least with the iPad, the 60-70 apps the wife and I already share on our iPhones (90% of them free, not one over $2 yet), would also work on the ipad for free, and the next 2 we buy in addition. Any app I'd likely buy for the iPad won't exceed $10. I might spend $30 on apps total over the life of my iPad, and I;d spend that on the Windows machine 3 times over before I even have it connected to WiFi at the house...